After the French Revolution, all of Europe was in turmoil. England had its hands full fighting Napoleon. England felt it could work with the Americans, and it was in its interests to allow them to "patrol the back door", as it were, and keep the French (and later, Prussians and Austrians) from flanking them in the New World. It freed their resources for the main battles in Europe itself.
Yes. The British were the only world power with a navy of significant enough strength to enforce the Monroe Doctrine. If the US tried to enforce the Monroe Doctrine, the Spanish fleet would have easily crushed the nascent US Navy.
Britain
President Roosevelt made it clear that he intended to enforce the monroe doctrine in 1904.
I'd go with France and Spain, the two European nations with the power and desire to continue colonizing the Americas. Ironically, the U.S. didn't have the naval power to enforce the Monroe Doctrine at the time in was enacted and was relying on Britain to do it for them--and Britain did, because they didn't want France or Spain to gain any more power.
doo doo butter
It didn't exist at the time. Mexico fought its war of independence between 1810 and 1821; the Monroe doctrine wasn't elaborated until 1823.
The Spanish-American War resulted from an attempt to enforce the principles expressed by the Monroe Doctrine. The United States sought to prevent European colonization and interference in the Western Hemisphere and saw Spanish control over territories like Cuba as a violation of this doctrine.
True
By using Its army to guard Latin America.
Even though British and US interests were at odds many times in the 19th century, Britain did support the Monroe Doctrine. In about 1820 the only colonial empire that could rival Britain's was that of Spain. Britain did not want Spain to reconquer its empire in the Americas, which had recently become independent. As a result, Britain supported the Monroe Doctrine in order to prevent this possible Spanish territorial reconquest.
To continue U.S. influence in Latin America
Monroe Doctrine 1823